Reactions. John Vincent Department of Chemistry University of Alabama

Transcription

1 Oscillating Chemical Reactions John Vincent Department of Chemistry University of Alabama

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3 Kinetics In kinetics we study the rate at which a chemical process occurs. Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs) Pearson Education, Inc.

4 Reaction Mechanisms The sequence of events that describes the actual process by which reactants become products is called thereaction mechanism Pearson Education, Inc.

5 The Collision Model Molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation Pearson Education, Inc.

6 Reaction Mechanisms The molecularity of a process tells how many molecules are involved in the process Pearson Education, Inc.

7 Multistep Mechanisms In a multistep process, one of the steps will be slower than all others. The overall reaction cannot occur faster than this slowest, rate determining step Pearson Education, Inc.

8 Oscillating Chemical Reactions Generally, in our experience, chemical reactions only overall go in one direction. Chemical reaction don t generally reverse course, or reverse course multiple times Discussion based on B. Z. Shakhashiri, Chemical Demontrations: A Handbook for Teachers in Chemistry, Vol. 2, University of Wisconsin Press, 1985.

9 Oscillating Chemical Reactions The oscillations in an oscillating chemical reaction are driven by a decrease in free energy of the mixture, as is the driving force in any spontaneous chemical reaction. Mostreactions just do not oscillate.

10 Oscillating Chemical Reactions Oversimplifying somewhat, all themolecules Oversimplifying somewhat, all the molecules in an oscillating reaction change their chemical identity simultaneously, at regular time intervals If the molecules are different colors, then a change of colors follows the rhythm of an oscillating reaction A new order seems to appears; coherence a communication among molecules

11 Far from Equilibrium System Spontaneous appearance of order unstable state certain fluctuations, instead of regressing, amplify and invade the system Catalytic loops Relationship to life? Prigogine & Stengers, Order Out of Chaos, Bantom Books, 1984

12 Three Common Features of Oscillating Reactions 1) The chemical mixture is far from equilibrium while the oscillations occur; an energy releasing reaction occurs that drives the oscillations. 2) Theenergy energy releasing releasingreactionreaction can follow at least two pathways, and the reaction switches from one pathway to another. 3) One of the pathways produces an intermediate, while another pathway consumes it. The concentration of the intermediateisis the trigger for switching from one pathway to another.

13 The Pathways When the intermediate s concentration is low, the reaction follows the intermediate producing pathway, leading to a relatively high concentration of the intermediate When the intermediate s concentration is high, the reaction switches to the consuming pathway, and the concentration of the intermediate decreases. Eventually the reaction reverts back to the producing pathway. The reaction then repeatedly switches from one pathway to another.

14 Simple Mechanisms for Sustained Oscillations Step 1: A +X 2 X Step 2: X + Y 2 Y Step 3: Y Q Intermediates are X and Y Steps 1 and 2 are autocatalytic their rates increase as the concentrations of the products increase (feedback) Sustained when using a large excess of A so [A]~constant

15 Belousov Zhabotinsky Reactions Carboxylic acids with common structure R C(O) CH2 C(O) OH Potassium bromate Manganese(II) or cerium(iv) catalyst Proposed mechanism much more complicated (about 30 species detected) Zaikin & Zhabotinsky Nature 225, 535 (1970) Field, Koros, & Noyes J. Am. Chem. Soc. 94, 8649 (1972)

16 Manganese catalyzes Bromate Malonic Acid Reaction My choice as very easy to assemble and works Just be careful to not contaminate with chloride ions See handout for procedure Cycles between colorless and the orange color of bromine

17 Basic Reactions Bromide ions are produced by BrO + 3 (aq) + 5 H (aq) + 4 Mn(II)(aq) HOBr(aq) + 4 Mn(III)(aq) + 2 H 2 O(l) Bromomalonic acidisformed is by CH 2 (CO 2 H) 2 (aq) + HOBr(aq) BrCH(CO 2 H) 2 (aq) + H 2 O(l) The bromomalonic acid is oxidized by Mn(III) releasing bromide BrCH(CO 2 H) 2 (aq) + 4 Mn(III)(aq) + 2 H 2 O(l) Br (aq) + HCO 2 H(aq) + 2 CO 2 (g) + 4 Mn(II)(aq) + 5 H +

18 Overall Reaction BrO 3 (aq) + CH 2 (CO 2 H) 2 (aq) H 2 O(l) + HCO 2 H(aq) + 2 CO 2 (g)